1. Field of the Invention
This invention relates to an ion source and a method of generating an ion beam using an ion source. In particular, the invention is concerned with an ion source having a plasma chamber which contains a cathode for generating a plasma in the chamber and which uses a magnetic field generator to produce a magnetic field extending in a magnetic field direction in the chamber. Ion sources are commonly used in ion implantation apparatuses in which a beam of desired ions is generated and directed at a substrate, typically a semiconductor wafer, for example a silicon wafer.
2. Background Information
It is a normal requirement in ion implantation, that ions are implanted uniformly over the surface of the semiconductor wafer. On the other hand, the ion beam impinging on the wafer during the implant process typically has a beam cross-section which is substantially less than the area of the wafer. To achieve uniform implantation over the wafer surface, various techniques are used to achieve a relative scanning action between the wafer and the ion beam.
A known type of ion implantation tool has an ion source which produces a beam containing ions to be implanted. The ion beam is directed through a region of homogenous magnetic field in an ion filter to provide spatial separation between ions in the beam with different momentum over charge (mv/e) ratios. A mass selector slit blocks any unwanted ions and allows desired ions to pass, optionally through an electrostatic accelerator, to a process chamber for implantation in semiconductor substrates or wafers. To improve productivity, a batch of wafers may be processed simultaneously by mounting them round the periphery of a process wheel mounted for rotation about an axis, so that the wafers on the wheel pass one after the other through the ion beam. The process wheel axis may at the same time be translated towards and away from the beam to provide a two-dimensional mechanical scan of the wafers through the ion beam, to provide the required uniform implantation over the full area of the wafers on the process wheel.
A further known type of implantation tool produces a so-called ribbon beam of ions, having a major dimension which is sufficient to extend right across a single wafer. A ribbon beam arrangement of this kind requires the wafer to be mechanically scanned only in one dimension, transverse to the ribbon beam plane.